Image-formation apparatus, controlling method thereof and image-formation method

1. An image formation apparatus comprising: at least one photosensitive drum; an optical writing device which carries out an optical writing process in a main-scanning direction on the outer surface of the at least one photosensitive drum at least line by line; a developing device which develops an electrostatic latent image optically written on the at least one photosensitive drum into a toner image by using toner; a paper conveyor belt which transfers a sheet to receive the toner image located on the at least one photosensitive drum, that is shifted in synchronism with the at least one photosensitive drum while being pressed onto the at least one photosensitive drum; a rotation angle detection unit which detects a rotation angle of the at least one photosensitive drum directly or indirectly; an eccentricity detection unit which detects an eccentric position from the rotary axis of the at least one photosensitive drum located in the center of a circle cross-section of the at least one photosensitive drum; and a correction unit which, based upon results of detection by the rotation angle detection unit and the eccentricity detection unit, finds an amount of eccentricity of the at least one photosensitive drum, an eccentric rotation angle and a radius of the at least one photosensitive drum, and based upon resulting values, compensates for a distortion and a color offset of a toner image before the transfer to a sheet.

2. The image formation apparatus according to claim 1 , wherein the correction unit finds an error between toner images at transfer positions of the sheet on the paper conveyor belt after a transferring process, the toner images being respectively derived from the at least one photosensitive drum in an ideal shape and the at least one photosensitive drum in an actual shape, and based upon the error, compensates for a distortion and a color offset of a toner image before the transfer to the sheet.

4. The image formation apparatus according to claim 1 , wherein the correction unit detects an error between installation positions of the at least one photosensitive drum and at least one of the optical writing device, developing device and paper conveyor belt, and based upon the error, corrects a distortion and a color offset in the toner image.

5. The image formation apparatus according to claim 1 , wherein the correction unit selects appropriate data from image data that modulates an exposing light beam outputted from the optical writing device and controls the data so as to correct a distortion and a color offset in the toner image.

6. The image formation apparatus according to claim 5 , wherein an optical writing position on the at least one photosensitive drum in a sub-scanning direction is fixed, and among interpolation image data that has been obtained by subjecting an original image data to a positional interpolation process at least either in the sub-scanning direction or in the main-scanning direction, the correction unit selects appropriate data and carries out a controlling operation.

7. The image formation apparatus according to claim 6 , wherein the optical writing device main-scans the at least one photosensitive drum by deflecting a light beam released from a light source by using a polygon mirror that is driven to rotate at a constant velocity by a polygon motor, with the optical writing position being fixed on the at least one photosensitive drum, and the correction unit outputs the image data in synchronized timing with a start of main-scanning by the polygon mirror.

8. The image formation apparatus according to claim 7 , wherein the correction unit carries out an optical writing operation by changing optical recording conditions in response to the peripheral velocity of the at least one photosensitive drum.

9. The image formation apparatus according to claim 5 , wherein the correction unit controls an optical writing position on the outer surface of the at least one photosensitive drum so as not to select the appropriate data of image data for modulating the exposing beam outputted by the optical writing device.

10. The image formation apparatus according to claim 9 , wherein a contact portion between the paper conveyor belt and the at least one photosensitive drum forms an apex on a round cross section of the at least one photosensitive drum in the paper conveyor belt direction.

11. The image formation apparatus according to claim 9 , wherein the paper conveyor belt presses a sheet of paper onto the at least one photosensitive drum to transfer the toner image formed on the at least one photosensitive drum.

12. The image formation apparatus according to claim 1 , wherein a contact portion between the paper conveyor belt, the sheet or the paper conveyor belt and the photosensitive drum forms an apex on a round cross section of the at least one photosensitive drum in an paper conveyor belt direction.

13. The image formation apparatus according to claim 1 , wherein the at least one photosensitive drum comprises a plurality of photosensitive drums aligned in one row on the paper conveyor belt in the shifting direction of the paper conveyor belt, and when toner images respectively formed on the photosensitive drums are transferred and superposed, the phases between the eccentricities of the photosensitive drums and positions of electrostatic latent images respectively formed on the photosensitive drums are made virtually coincident with each other with respect to the photosensitive drums so that the phases in variations in density of transferred images caused by the eccentricities are made coincident with respect to the photosensitive drums.

14. The image formation apparatus according to claim 1 , further comprising an image reading device which reads an image of an original, wherein image formation is carried out based upon the image data thus read.